The present invention relates to a method and apparatus for multi-level tone display for a liquid crystal display apparatus.
A liquid crystal display apparatus disclosed in Japanese Patent Application Laid-open No. 59-149393 accomplishes three-level tone display by equally splitting the voltage pulse width in one horizontal period so that the voltage pulse applied to liquid crystal has its effective value varied. The conventional technique will be described with reference to FIGS. 1 to 3.
FIG. 1 shows an example of the liquid crystal display apparatus which accomplishes three-level tone display by controlling the pulse width, in one horizontal period, of the voltage pulse applied to the liquid crystal apparatus. A control circuit 100 generates two kinds of data of display information a and b for displaying a liquid crystal dot in one horizontal period. As seen in FIG. 2, the data a and b are not simultaneously supplied, but rather the data a is supplied during a first half of one horizontal period and the data b is supplied during the second half of the horizontal period. An X drive circuit 115 introduces data a in response to a data latch clock 7 and, after the drive circuit has introduced display data for one line by repeating the same operation, outputs liquid crystal application pulses corresponding to the data a onto signal lines X1, X2, . . . Xi in response to a pulse clock 10 in the order of introduction. The pulse clock equally splits the line clock 9 in every horizontal period. Next, in a similar manner, the X drive circuit introduces the data b in response to the data latch clock 7, and outputs liquid crystal application pulses corresponding to the data b onto the signal lines X1, X2, . . . Xi in response to the pulse clock 10 in the order of introduction. A Y drive circuit 116 introduces a leading line clock 8 in response to the line clock 9 to produce a high state at line Y1, and in response to the successive line clocks 9 it shifts the high state to Y2, . . . Yj. A liquid crystal panel 117 is an i-row by j-column matrix panel, and liquid crystal cells with the application of high-state pulses among X1, X2, . . . Xi from the X drive circuit 115 and Y1, Y2, . . . Yj from the Y drive circuit 116 are activated for display.
FIG. 2 shows the liquid crystal application pulse produced by the X drive circuit 115. During one horizontal period, the X drive circuit receives the display data a during the first half of the horizontal period and the display data b during the second half, and delivers one of four kinds of pulses, i.e., pulse 1 through pulse 4, selectively depending on the data a and b. FIG. 3 shows the correspondence between the display data a and b and the pulse from the X drive circuit.
With the display data a and b generated by the control circuit 100 being 0 and 0, the X drive circuit 115 delivers pulse 1 as a liquid crystal application pulse, and a display dot is inactivated. With the display data a and b being 1 and 1, the circuit 115 delivers pulse 4 to activate a display dot. With the display data A and B being 0 and 1, or 1 and 0, the circuit 15 delivers pulse 2 or pulse 3, respectively, producing an intermediate tone display between an active and inactive dot in both cases. The brightness (transmission factor) of the liquid crystal is dependent on the effective value of the voltage applied to it. Since the pulse clock 10 is obtained from equal splitting or division of the line clock 9, the pulse 2 and pulse 3 have an equal H period and thus an equal effective value in voltage. Consequently, the pulse 2 and pulse 3 provide an equal brightness of liquid crystal, which is an intermediate brightness between an active dot and inactive dot, resulting in the accomplishment of three-level tone display. Accordingly, the liquid crystal display apparatus shown in FIG. 1 is capable of tone display by varying the effective value of the voltage applied to the liquid crystal panel 117 through the combination of display data a and b, and it accomplishes intermediate tone display.
The foregoing technique as described in the aforementioned publication accomplishes intermediate ton display by splitting the voltage pulse width applied to the liquid crystal in one horizontal period equally into two and combining the pulses. However, the aforementioned publication does not disclose how to produce the data a and b from the display signals and describes the data a and b as corresponding to only one picture element of the liquid crystal panel. Generally, the liquid crystal panel is provided with many picture elements in a horizontal or raster direction. Thus, in practice it is difficult to produce the data a and b of one raster from display signals. Each display signal supplied to a liquid crystal display apparatus has a pulse width of one horizontal period.
Reference is made to FIG. 4 which shows an example of a liquid crystal panel of a liquid crystal display apparatus having four dots (picture elements) in the horizontal direction with X lines X1, X2, X3 and X4 and two dots in the vertical direction with Y lines Y1 and Y2, and which may be utilized for intermediate tone display in accordance with color display signals R, G and B as shown in FIG. 5 wherein a pulse width is split for one horizontal period. As shown in FIG. 4, the color display signals R, G and B will be displayed by an OFF state, an intermediate state and an ON state. On line Y1, pulses 1, 2, 3 and 4 from the lines X1, X2, X3 and X4 are respectively supplied to the display apparatus so that the OFF state, the intermediate state, the intermediate state, and the ON state are respectively displayed by the panel. Further, on the line Y2, pulse 2 is supplied from each of the lines X1-X4 so that the intermediate state is displayed. If in this configuration, the intermediate tone display method with splitting of the pulse width is utilized, the data a for displaying the line Y1 must be (0,0,1,1) and the data b therefor must be (0,1,0,1). Also, the data a and b for displaying the line Y2 must be (0,0,0,0) and (1,1,1,1), respectively, as shown in FIG. 5. Therefore, if the intermediate tone display with splitting of the pulse width is applied to a liquid crystal panel of 4.times.2 dots, it is necessary to produce the data a and b as shown in FIG. 5 from the original display signals, but since the aforementioned publication only discloses a one dot display, such publication provides no description concerning the necessary operations. Additionally, when the display area for intermediate tone display with the same combination is expanded in the X direction, a rising edge or falling edge of pulses which make a simultaneous transition in one horizontal period create a noise, resulting in a degraded brightness of display. Also, when the display area for intermediate tone display with the same combination is expanded in the Y direction, the liquid crystal application pulse has its frequency component raised, resulting in a degraded brightness of display and increased crosstalk. Moreover, liquid crystal display based on the conventional technique merely accomplishes three-tone display that is an active dot, intermediate dot and inactive dot, and it is limited in ability in a sense of a multi-level tone display.